1. Field of the Invention
This invention relates to the forming of sheet material, including sheet material which is stretchable both axially and transverse the axis thereof, and in particular to an improved sheet forming machine and method of making such a machine.
2. Description of the Prior Art
It is known in the art to provide a forming machine for sheet material and method of making such a forming machine wherein the sheet material is advanced serially through a succession of working stations for physical modification of such sheet material and with the machine comprising, a pair of spaced apart rails extending along the stations, a pair of first clamp assemblies each mounted on an associated one of said rails for reciprocating movements therealong, with the first clamp assemblies being adapted to grasp opposite side edge portions of the sheet material and upon moving the first clamp assemblies in one direction along the rails advance the sheet material through the stations, means for moving the first clamp assemblies along the rails, a plurality of second clamp assemblies mounted on the rails in longitudinally fixed relation with each of the second clamp assemblies having an upper and a lower clamp member for grasping an associated side edge portion of the sheet material transversely inwardly, independently, and in a non-interfering manner with an associated one of the first clamp assemblies and such a machine and method of making same are disclosed in U.S. Pat. No. 3,425,611.
However, a machine as disclosed in the above-mentioned patent has certain limitations in that it is slow in operation, uses a plurality of second clamp members of the character mentioned above along each rail which grasp only small axial increments of the side edge portions of the sheet material, and lacks apparatus for precisely adjusting and holding the rails. The grasping of only small increments of the side portions often results in slippage of the sheet material during physical modification thereof and in addition to slippage the physical modification of shrinkable plastic material, for example, may result in substantial distortion during such modification. However, any slippage or distortion (such as by stretching) at a working station results in misalignment at a subsequent station whereby an article such as a package made in the machine disclosed above may have poorly defined portions or parts.